Wireless signaling system



m6. 6, 1949 I F, c, WILLIAMS 2,4$0,3$4

WIRELESS SIGNALING SYSTEM Filed July 25, 1947 s She ets-Sheet 1 1949 F. c. WILLIAMS WIRELESS SIGNALING SYSTEM 6 Sheets-Sheet Filed July 25, 1947 ACNE :06

A on? mom? lnvenlor B r. 0. William Attorney 6, i'w- F. c. WILLIAMS WI RELESS SIGNALING SYSTEM 6 Shee ts-Sheet 3 Filed July 25, 1947 Dec. 6, 1949 F. c. WILLIAM 2,490,394

WIRELEESS SIGNALING SYSTEM Filed July 25, 1947 I 6 Sheets-Sheet 4 3 6, 1949 l F. WILLIAIMS-Y I 2,490,394

f WIRELESS SIGNALING SYSTEM Filed July 25, 1947 I 6 Sheets-Sheet 5 I lnvenlor By P WIRELESS SIGNALiNG SYSTEM Filed July 25; 1947 I s Sheets-Shet .6

M M) vo 2-5 5 7'5 10 -7 I I f T lN/ SECS sine c [new -kO$6COS case-cow s/ha cos Inventor F. C- IiILlh-ma A Nor/my Patented 6,

OFFICE wmnmss SIGNALING SYSTEM Frederic Calland William's, Timperlem England Appliciigion July 25, 1947, Serial Great Britain June 14,1945

/ Section 1, Public Law 690, August a, 1946 Patent expires June 14, 1965 17 Claims.

This invention relates to wireless signalling systems and is more particularly concerned with arrangements using a pulsed signal wave.

One particular aspect of the invention relates.

to arrangements for aiding the navigation of mobile craft, such as aircraft or shipping, employing pulsed transmission from a number of spaced fixed aerials and in which the different transit times taken by such pulse signals to'travel from the various spaced aerials to a distant mobile craft serve to provide information in the latter of its bearing or'position with respect to the known location of said fixed aerials. Examples United States applications Serial Nos. 527,017 and 527,018. I

According to one feature of this aspect of the invention information is provided in a mobile craft as to its bearing direction with respect to a ground beacon transmitter at a known geographical position by the radiation of pulse signals, at a constant pulse repetition frequency, in a series of groups in predetermined regular order from two or more suitably spaced ground transmitting aerials at said known location and by the comparison, at said mobile craft, of the phase relationship (at the pulse repetition frequency) of the pulses of the pulse groups received from one ground aerial with respect to those of pulse groups received from another fixed ground aerial.

The ground beacon transmitter may comprise two aerial pairs disposed in lines which are at right angles to one another. In one preferred arrangement three aerials are employed and are arranged respectively at the corners of a rightangled isosceles triangle. With this particular arrangement the pulse groups are radiated in cyclic order from the apex aerial, one base corner aerial, the apex aerial and then the opposite base corner aerial and so on. In an alternative embodiment four aerials are employed arranged respectively at the corners of a square, the pulse groups in such an arrangement being radiated in cyclic order from one corner, the diagonally opposite corner, a further corner and then its diagonally opposite corner and so on.

According to'another feature of this aspect of the invention a wireless signalling system of the of signal currents related to said different transit of such systems are to be found in copending times, is characterised in that the various signals resulting in said opposed signal currents are arranged to traverse a common signal channel until they have been given the' final form suitable for effecting direct operation of said meter device. In this way errors due to unbalance of parallel signal handling channels are avoided "Another aspect of the invention concerns a phase discriminator arrangement particularly,. although by no means excl sively adapted for use in apulsed wireless signa liing system for navigational aiding purposes as previously referred tm According to this aspect ofthe invention a phase discriminator arrangement for use in a wireless signalling system employing pulsed signals conveying intelligence by phase variation of their pulse recurrence frequency includes an oscillator'arranged for operation at the mean pulse recurrence frequency but capable of variation over a chosen frequency range centred about said mean frequency by the application of an appropriate controlling voltage thereto and phasecomparing means for determining the phase relationship between applied input pulse signals and an oscillation derived from said oscillator and providing a controlling voltage to said oscillator which tends to maintain the aforesaid phase relationship at a chosen value, said controlling voltage serving also to provide, by its variations, an output component related to the conveyed intelligence.

Yet a further aspect of the invention relates to a wireless signalling system wherein information or intelligence is derived at a receiving station from the arrival timing of pulses of a received pulsed signal wave. According to this aspect of the invention interference effects due to spurious and other signals, such as those arising from ground reflections are reduced or eliminated by the provision of means for initiating derived pulses of predetermined amplitude and time duration by the reception of a chosen leading-edge portion only of the received signal pulses and by the utilisation of such derived pulses instead of the actual received signal pulses for the derivation of the required information or intelligence.

In order that the various features of the inven tion may be more readily understood a number of embodiments thereof will now be particularly described with reference to the accompanying drawings in which Figure 1 is a schematic lay-out diagram of one form of ground transmitting or beacon station,

Figure 2 is a block schematic diagram of one form of mobile craft apparatus.

Figures 3, 4 and 5 are circuit diagrams of'certain portions of the apparaturs shown in Figure 2. Figures 6, 7, 8 and 9 are graphical waveform diagrams illustrating the operation of the mobile H. T. supply arrangement,

are summarised in Fig. 8 (ii) which is a plot of the time difference between the incoming 5 kc./s.

fFigure 11 is a fragmentary circuit diagram of a modification,

Figure 12 is a schematic lay-out diagram,.similar to Figure l of a modified. ground transmitter arrangement, while Figurel3 is a'similar schematic lay-out "an gram of a further modified ground transmitter arrangement.

Referring first to Figure 1, the ground transmitting or beacon station comprises three aerials A, Band C each having arr-omnidirectional radia-.'

tion characteristic and located respectively at the corners of an isosceles triangle whose equal sides" are of'length I. These aerials are supplied through equal electrical length feeder lines I! [by a single transmitter ill. The aerials are connected' to the transmitter in'tum .by a rotary switch I i in the cyclic order A, B, A, C, A, B, A, C and so on. a

The transmitter l0 generates short pulses of RF power of the order of, say 1.5 microseconds duration at a constant repetition frequency of,

say, 5 kcs./s. The switch H is arranged to oper ate regularly at, say 3000 R. P. M. with the result that, assuming perfect switching, groups of 25 pulses are radiated from each aerial in turn in the cyclic order stated above. Each group cycle A,

I B, A, C will be recurrent 50 times per second while each group will last for 0.005 second. The radiaa beacon transmitter arrangement by a mobile craft, e. g., an aircraft MC which is located at a bearing angle 0 with reference to the direction through aerial A and B. The latter direction is conveniently, although not necessarily, one pointing due north. The aircraft will be assumed to be so remote from the beacon that the direct paths A-MC, B-MC and C--MC can be regarded without significant error as being parallel.

At the mobile craft there will be received the groups of pulses radiated in turn from the aerials A, B and C at the chosen 5, kc./s. recurrence frequency. As the pulsesare switched from aerial A to aerial B there will be an'apparent change in the phase of the new (B aerial) series of pulses relative to the old (A aerial) series because the first pulse of the series from aerial B will reach the mobile craft sooner, by a time proportional to Z cos 0, than if it had been radiated from aerial A. Similarly, upon reversion to aerial A, 0.005

second later, there will be a further phase-change as a strobe.

pulses at the mobile craft MC and a steady series in phase with those from aerial A.

Referring now to Figure 2 which shows schematically the mobile craft equipment, pulse signals from'the ground beacon are received by .aerial 13, are suitably amplified and demodulated in receiver which provides a rectified pulseform output for application to a pulse generating unit l5-by which a 5 microsecond pulse is generated for each received pulse signal.

These 5 microsecond pulses are arranged so as to be initiated in response only to the first 0.25 microsecond period following the leading edge of the related received pulseapplied from receiver l4.

Associated with this unit I5 is an automatic volume control circuit it which serves to provide a controlling voltage which stabilises the gain of receiver I4 at a .chosen level. This AVC unit is arranged to be quick acting whereby it brings the amplitude of the pulse output of receiver H to the chosen levelwithin a few, e. g., three, pulses of the beginning of any series of similar pulses.

U .it I! is an oscillator having a natural or freerrunning frequency substantially equal to the chosen pulse-recurrence frequency of the cooperating ground beacon e. g., 5 kc./s. This oscillator is arranged, however, to be varied continuously in the absence of any received signals, over a fre-' quency range of 11% e. g., from 5050 to 4950 c./s. by a reactance valve unit l8 which is, in turn, controlled by a saw tooth control waveform provided by a sweep valve unit I 9. The natural sweep period is of the order of a few seconds.

The oscillator unit I! also controls a pulse generating unit ill which provides a 10 microsecond pulse accurately time, related with each oscillation of the oscillator l1 and therefore varying in phase with respect to the incoming signal pulses. Sucha pulse is hereinafter referred to These 10 microsecond strobe" pulses are fed -back to the 5 microsecond pulse generator unit l5, where they serve to bring that unit, which is normally held inoperative, into operation during the actual pulse periods. By this arrangement interference by signals arriving at times other than those corresponding. to the anticipated arrival times of signals at the chosen pulse repetition frequency of the ground beacon station is avoided.

These 10 microsecond strobe pulses are also applied to the sweep Valve unit l9 where, upon the occurrence of a chosen time relationship thereof with the5 microsecond pulses from unit l5 which are time-related to the received signals, the natural sweeping action of unit I 9 is suspended and the oscillator I! thereby kept at a chosen frequency. This effect clearlyoccurs only when the frequency and phase of the pulses derived from oscillator H coincide with the frequency and timing of the received signal pulses'so that the arrangement is effectively one which will search for and then look onto the signal pulses from the chosen ground station. Any phase discontinuity-in the latter pulses is equivalent to a temporary change of frequency and in the process of re-establishing the locked condition above referred to the sweeping valve unit l9 pro vides a. control waveform for the reactance valve is of a form that bears a linear relationship to the sineand cos-phase-change values already is repeated continuously. These phase-changes referred to.

-- is followed by the oscillator I'I within 0.0025's'ec-J from'5 kc./s. by the reactance valve l8 fora 'sufficienttime to allow theintegral. of the .change This follows from the fact that the phase-differsin outputs may be applied in turn to'a centre- 40 as indicated, and since the same channel is used of the sin/cos potentiometer is provided with s \1 I It H The circuit constants of the synchronising-ar ground beacon, may be read off dlrectly'from rangement thus provided-are such that any small 1' discontinuity in phase of the incoming signal big'uity can usually resolved in a number of ways. The potentiometer calibration dial conond. In order to perform this repha'singf the, ,veniently consists of a calibrated ring surroundfrequencyof the oscillator I! must be shifted in the centre zero meter 24.-

The. width, modulated received signal pulses f fr y o equal t required Ph s hif the 5 kc./s. recurrence frequency and the 50 c./s. I l0 aerial switching frequency components and ence between two waves initially n phase i's'the passes the remaining width modulation comintegral of their frequency difference. ponents (which may constitute a communication It follows from the above that when the phase channel) to an amplifier 21 for ultimate operaof the received signal 'ch as Shown 1' tion of a sound reproducer such as head teleure 8 (ii) the frequency of the oscillator will '15 p 2a,

. change as indicated in Figure 8 (iii)-by virtue of i, The receiving aerial l3 may be of any suitable form and is conveniently of omnidirectional character.- The receiver 14 is of conventional form, more particularly of the type used in connection with pulse modulated radar systems. In

one practical embodiment it had a sensitivity of 80 microvolts and a bandwidth of 4 mc./s. at an operating frequency in the region of 200 mc./s.

- A suitable form of the 5 microsecond pulse generating unit 15 and the A. V. C. unit I6 is shown in Figure 3 where valves V1, V2, V3 and V4 with their associated diodes D2, D3, D4 and D5 and other circuit components constitute the pulse generating unit and valve V5, with its diode D1 and associated components, the A. V. C. unit.

The output from receiver 14, in the form of the operation of the automatic synchronising means 1 It further follows from the fact that phase is the integral of frequency that the areas under the pulses of change of frequency shown in Figure 8 (iii) are proportional to 1 sin 0 and to 1 cos 0 for both, the constant of proportionality is the same for the sin 0 and cos 0 terms. There is thus in the circuit, a voltage proportional to frequency which controls the reactance value unit [8. 1 a

A version of the. control waveform shown in Figure 8 (iii) is applied, after amplification in a linear amplifier 2| to a sin/cos potentiometer 22 by which two anti-phase versions of such control waveform at amplitude levels proportional respectively to the 'cosine and the sine values of the angular control setting of the potentiometer are made available. These two outputs, which are illustrated graphically in Figure 9 (iv) and (v) are applied to a display unit '23 which is effectively a switching device by which selected portions of each of said cos and minal tl to the control-grid of valve V1 whose anode circuit is completed by way of the parallel paths through valves V2 V3. The suppressor grid of valve V1 is supplied by way of terminal t2, with the strobe pulse waveform, derived in a manner described in detail later, comprising a series of positive-going pulses at the chosen pulse recurrence frequency and each of 10 microseconds duration; Except during the time of such positive going pulses, valve V1 is cutoff at its suppressor grid and in consequence, any received signals will be quite ineffective to cause interference by spurious operation of the system.

When, as will be the case in proper operation, the received signal is applied to the control-grid of valve V1 whilst the suppressor grid thereof is lifted to earth -potential, then valves V1 and V2 take current, valve V3 remaining cut-off by virtue of the greater bias potential upon its control grid. Amplified negative-going pulses therefore appear at the anode-of valve V2. By reason of the feedback path from anode to control-grid of valve V1, the overall gain is stabilised at a valve determined by the ratio of resistances R1 and R2 and is, in the example given, equal to 5. The negative pulses at anode of valve V2 are applied through diode D2 to control-grid of valve V4 where they cause cut-off of anode current and consequent ringing of tuned circuit L. C. coupled to the valve anode circuit. The product I of circuit components L. C. is such that, for each ringing operation, a 5 microsecond positive pulse is formed first, followed by a similar negative swing. The latter is absorbed by diode D3 and suitably calibrated setting means relating -the the former'applied by way of terminal t: to sweep ground beacon bearing angle (0+1r) to the values valve unit is, p o d thereby so t by u in the po- The anode circuit of valve v, includes a retentiometer t0 t e q d bearing angle a 70/sistance R: which, in conjunctidn with condenser then navigating the craft so as to maintain a Q, causes the production of I a positive-going zero meter indication, a straight course along such bearing direction may he travelled. Alterconstant of network Re, C1 is such that the natively, by adjusting the potentiometer to obtain potential rise ofpoint a, which is applied to a zero meter reading, the bearing angle of the 75 control-grid of valve Va. is sumcient to open zero indicating meter 24. The switching function of this display unit 23' is controlled'so as to select the sin 0 cos 5 pulses of waveform, Figure 9 (iv) and the cos 0 sin pulses of waveform, Figure 9 (v), once during each cycle of the ground station aerial switching. Since this switching function must clearly be time controlled to synchronise with the ground beacon aerial switching cycle the necessary control for the display unit 23 is provided by applying the received signal pulses to a reference unit 25 where, by resolving the 50 c./s. width modulation imposed upon the rear edges of the received pulse signals, a 50 c-./s. sine waveform is .derived. From this waveform, shown in Figure 9 (i), there are developed two anti-phase square pulse switching-waveforms, shown in Figue 9 (ii) and (iii),-whose respective positive going pulses are appropriately timed to select the above stated portions of the two waveforms of diagrams (iv) and (v) Figure 9 applied to the display unit 23 from the sin/cos potentiometer 22.

The resultant meter indication is proportional to sin (95-0) and will become zero or balanced when 0 equals or +l80. The control shaft the calibrated scale) e The possible amare also applied to a fllterunit 28 which rejects positive-going pulses, is applied by way of ter-- sawtooth waveform at the point a. The timepulses.

second after the instant of original application of the related pulse to terminal ti from the receiver l4. Valve V3 thereupontakes the current through valve V1 and valve V2 is cut-oft for the whole of the remaining period of the applied signal pulse. The rise in anode potential of valve Vi does not reach valve V4 owing to the blocking action of diode D2. v

The anode waveform of valve V2 is also applied to-diode D1 and the rectified currentproduced operates valve V5. ceived signal pulses increases an increase occurs in the mean rectified current flowing 'through resistance R4 which causes a discharge of the feed back condenser C2 and invokes a fall in potential at the anode of the valve. Since this anode potential is that which is applied by way of terminal 154 to the screen-grids of some of all of the I. F. valves of the receiver H (Figure 2),

strobe pulse generating unit 20 is shown in Figure 4, where valves Va, V95, Vilb and V109. with their associated diodes D8, D9 and other circuit components constitute thesweep valve unit I! while valve V1 with diode D6 and other elements forms the 'reactance valve unit I8. Valve V6c and diode D1 constitute the 5 kc./s. oscillator unit I! and valve V6b the strobe unit 20.

If the amplitude of the re-,

there is a consequential. reduction of the receiver a gain to stabilise the receiver output at the chosen level. If the received signal amplitude decreases a reverse action takes place.

Valve V3, as already stated is turnedon 0.25 microsecond after the front edge of each received signal pulse and is turned off again at the end of each signal pulse by the cutting-off of valve V1 in series therewith. Since the duration of the various signal pulses is determined by the width modulation of their rear or trailing edges, the anode waveform of valve Va will comprise a series of negative-going pulses. still carrying the width-modulation of the original signal This anode waveform is applied by way Valve V8 is arranged as a saw tooth waveform generator having a cycle period of the order of a few seconds. Its operation in the absence of a receivedsignal input is briefly as follows. "At the start of a cycle, the anode of V0 is at a voltage level, say +250 v., set. by'resistance network R5, R6 and R1. The control grid and the suppressorgrid of the valve have previously been biased negatively due to connection of resistance Ri0 to a 150 v. supply by relay RL], but on account of the deeenergisation of such relay, resistance R10 has just been connected to earth. Due to current flow through resistance R8, the control grid potential rises and the anode voltage runs down, being linearised by feed back network C3, C4, B9. run down period the screen current increases, cncrgising relay RL1 and so bringing about reconnection of resistance RN to the l50 v. supply. This causes cut-off of anode current and the charging of the feed back network through resistance R5. When the charging current begins to fall the' control grid potential starts to move a negatively whereby the screen current also falls and eventually de-energises relay RL1 to recommence the sweep cycle afresh.

of terminal is to the reference unit 25 and filter unit 26 of Figure 2.

By the operation of the arrangement so far described both the A. V. C. level and the derived pulses, applied by way of terminal is to sweep unit 19 and used ultimately to provide the required bearing information or intelligence, are dependent upon and influenced only by the first 0.25 microsecond period of each'received signal pulse. By this means any interfering signals, particularly those due to reflections from ground objects, which arrive more than 0.25 microsecond after the front edge of each pulse are neglected. This increases the freedom of the arrangement from site errors. The manner ofoperation is illustrated by Figure 6 where, in diagram (1) P indicates .a typical received signal pulse as applied from the receiver M to valve V1. The A. V. C. system operates to maintain the signal at 30 v. amplitude at the instant 0.25 microsecond after the inception instant thereof. The potential level required to open the control-grid of valve V1 is set at one-third of the A. V. C. level, i. e. 10 v., whereby the derived pulse available at terminal if: and shown at dp in diagram (ii) commences at that instant (a2) and lasts for the chosen duration of 5 microseconds. 'The instant as is defined as the instant of arrival of the pulse. Fluctuations of the received pulse amplitude on its duration subsequent to the 0.25 microsecond point, as illustrated at P1, 1 2, pa in diagram (1) are entirely without effect upon the derived pulse.

The output pulse from anode of valve V3, on

The reactance valve V1 is supplied on its control grid with a proportion of the anode wavewform of valve V8 from the junction of resistors R6, R1 whereby it acts as a capacitance in shunt to the tuned circuit L1, C1 of the oscillator valve V6a, the value of such capacitance being lineraly related to the applied saw tooth control waveform from the sweep valve V8. In practice the application of this control voltage, in the absence of a received signal, causes the oscillator fre-' quency to vary' linearly between the limits of 5055 c./s. and 4950 c./s.

The oscillator valve VBa is of the Reinartz type but has a. cathode load and a long time constant network C1, RM in its grid circuit. The upper limit of the grid voltage swing is set at +60 v. by the diode D1 whereby an output in the form of positive pulses approximately 60 'v. in amplitude and 20 microseconds in width are obtained at the cathode end of resistance Rl5. These pulses are applied to the grid of valve Vfib by way of the network of condenser C8 and resistance RIG. The negative-going trailing edge of each pulse serves to cut off the control grid of valve Vfib and thereby to produce at its anode positive-going pulses which have a width of 10 microseconds set by suitable choice of the time constant of network C8, RIG. These pulses are A. C. coupled by condenser C9 and resistance RI! about a potential level of v. and are applied through integrating network CIO and RIB to the control grid of valve V92. As a result, a rising saw tooth voltage is applied to the grid of valve V98. for the 10 microseconds ofeachpulse provided by valve Vfib. The square positive-going 10 microsecond pulse from the anode of valve Vsb' is also applied by way of ter Towards the end of the is, however, Supplied at such control-grid with.

the-' microsecond pulses generated by valve V4, Figure 3, in response to each received signal pulse. When such 5 microsecond pulse is so applied to valve VlOa, current normally flows through this valve and through valve Veb. If the oscillator valve v68 is properly locked or synchronised with the received signals then the rising voltage applied from the anode of valve Vse reaches a level at which valve V95 is turned on at a time 2.5 microseconds after the initial edge of the 5 microsecond pulse applied to valve V109. so that,

for the remaining 2.5 microseconds of the 5 microsecond pulse period, current flow is through valve V98. instead of through valve Vat. These conditions are illustrated in Figure 7, where diagram (1) shows a received pulse signal with its rear edge width modulated as shown by the shaded region; diagram (ii) the microsecond pulse generated at the anode of valve Va, 1. e., the strobe pulse; diagram (iii) the 5 microsecond pulse generated in unit l5 from the received signal pulse of diagram (1) and diagram (iv) the rising sawtooth waveform derived from the pulse of diagram (ii) and applied to valve V98- Under these conditions the current flow through the primary winding of transformer of TI is reversed at an instant substantially coincident with the centre of the 5 microsecond pulse period. This is illustrated in diagram (v) of Figure '7.

The secondary windin of transformer TI is arranged to apply its output through reversed diodes D8, D9 to charge a condenser CH which is. also connected to the control grid of the sweep valve V8. Normally each of these diodes is biased 03 since the control grid of V8 is approximately at the potential of 4 v. whereas the anode of D9 is connected to a potential supply of 8 v. so that the anode of D8 is four volts below its cathode and the cathode of D9 is four volts above its anode. When the current flow through transformer TI is reversed at the midpoint of the 5 microsecond pulse period as in diagram (v),

lug potentials of valve Va by the current derived from condenser Cl l. .The oscillator valve Vila is then held locked to the incoming received pulses.

ceived pulses, i. e., upon switch g from one ground aerial to another the aforesaid balance Figure '7, the effective voltage across CM is zero and no mean current reaches the control grid of valve V8. Q If, however, the change over point is displaced to one side or the other of the midpoint, due to relative phase displacement between the received pulse signals and those due to oscillator valve Vea, then the condenser CH acquires a voltage of certain implitude and polarity and this is in turn applied to the control grid of sweep valve V8 where it causes a related change of anode potential to cause reactor valve V! to alter the frequency of oscillator V68 so as to restore the previous condition of synchronism. In

sweep valve V8 and reactance valve V'l until 00- incidence in the manner above referred to is obtained. When this occurs the particular frequency at which the oscillator is then functioning will be held by cancellation of the sweep.-

of current flow in the anode circuit of valves Via and vllb will be disturbed and the sweep valve circuits Va appropriately effected to alter the oscillator frequency to re-establish locking. Since only the phase of the leading-pulse of each pulse group is altered relative to the last pulse of the previous group the aforesaid frequency change to re-establish locking will be followed by a further change of the oscillator frequency'back to its original value. Thisis accomplished by a further control potential provided by the sweep valve V8 in similar manner to that already described. A representative waveform at the anode of valve Va for one aerial switching operation is shown to an enlarged scale in diagram (i) of Figure 8, while diagram (iii) of that figure shows an example of one complete cycle of aerial switching.

Since out of balance currents due to the relative phases of the signal and strobe voltages are integrated in the sweep valve circuit to produce the control voltage and since the control voltage changes the frequency of the oscillator, which in turn changes the phase, the circuit is of the feed back type with two integrations in the feed back loop 1. e., the integration from current to voltage and the integration from frequency to phase. Hence the circuit has a natural period of oscillation and its resonance to impulses will be similar to that of an LC circuit. Intentional impulses, i. e., the phase changes of cos 0 and sin a previously mentioned are applied to the circult and its response is controlled by the damping component R8 between the anode and control grid of valve V8. In practice, the circuit is so arranged that the base of each pulse of this con- .trol waveform, occupies approximately 45 of this figure, valves Vlilb, vaavm, V16a, Vmb with their associated components constitute part of the reference unit 25, valves Vm, V m, VlBa and Vl8b form the display unit 23 and the potentiometer SCP with its associated transformer, the sin/cos potentiometer unit 22.

The amplified control waveform of Figure 8 (iii) is applied by way-of transformer T2, having an earthed center tapped secondary winding to opposite points of a toroidal potentiometer SC P. The windings of this potentiometer are earthed at diametrically opposed points as shown while opposite points displaced therefrom are connected respectively to the opposite ends of the secondary winding of transformer T2. The two moveable.contacts on the potentiometer are displaced 90 with respect of one another whereby they provide voltage outputs which are proportional to cos and sin of the input waveform to transformer T2 where 4 is the angle of displacement of the controlling shaft for the sliders from a zero position. These output waveforms other will allow conduction of the current pulses thereof to the meter 24 whatever their polarity may be.

The ---sin c output from the potentiometer SC? is similarly applied to valves V18a,vl8b which are likewise controlled by a switching waveform supplied from reference unit 25 by way of secondary windings S3, S4 of transformer T4.

The switching waveforms for these valves V11, V11 must be in precise synchronism with the aerial switching cycle of the ground transmitting stationand are obtained in the following manner. The width modulation of therear edges of the received signal pulses are extracted by any suitable means forming part of the reference unit 25 (but not shown) and the resulting, e. g., 50 cycle, sine wave applied to valve Vlflb which operates as a paraphase amplifier to drive valves Vla, V165. in push-pull. The latter valves are arranged to effect squaring of the applied sine wave oscillations whereby they each produce a square pulse waveform at their anodes comprising one negative-going pulse per 'cycle having a duration approximately equal to 90 of the cycle. The pulses of the two waveforms are 180" phase displaced with respect to one another and are applied by way of cathode follower valves V1513, Vlfib to the primary windings of transformers T3 and T4 respectively.

The secondaries of transformers T3, T4 are so connected that the similar but positive-going pulse waveforms, shown respectively in diagrams (ii) and (iii) of Figure 9, are applied to the related gridsof the valves VI! and Vl8, so-that the valves Vi'h, Vl'lb are opened for a chosen 90 cycle period during each aerial switching cycle and the valves Vlfla and V|8b similarly opened for another 90 period 180 later in each aerial switching cycle. As already described and as illustrated by comparison of diagrams (ii) and *sin 0 cos output pulses of waveform (iv) and the cos 0 sin output pulses of waveform (v) are passed to earth through the meter 24 which is shunted by a large condenser whereby integration'takes place and an indication proportional to the mean current is obtained.

It will be noted that in the above described arrangement the final bearing is obtained by comparing the amplitudes of two pulses, and that these two pulses traverse the same network right through to the sine/cosine potentiometer. The only double-channel part of the system comprises the. valves Vin,- 'Vrm, Vina and VlBb and the resistors in series therewith. These resistors are chosen so as greatly to exceed the resistance of the anode-cathode paths of the valves when conducting, and to retain symmetry between the two channels it is necessary only to ensure that the two resistors are equal.

The phasing of the switching waveforms shown in diagrams (ii) and (iii)v of Figure 9 is not critical; provided the selected pulse is entirely contained within the period of the operating pulse, small Phase variations of these waveforms are irrelevant. It is for this reason that the switching waveforms pulses are desirably arranged so as to be maintained over a cycle period equal to whereas the controlling waveform pulses themselves have a duration equal to about 45.

Since the channel from the receiver to the in dicating meter is opened up only during the 10 microsecond strobe pulse periods 1. e., for a total of 5000 10 10- or 0.05 second in every second, an appreciable saving in H. T. supply to the associated receiver can be achieved by similarly limiting its application to short pulse periods coincident with said strobe pulses. Figure 10 shows a fragmentary circuit diagram of one arrangement for this purpose.

Referring to this figure, valve V20 is arranged as a blocking oscillator which is appropriately triggered by way of lead 30 from a suitable point of the oscillator circuit of unit II, Figure 2,,or more conveniently from the strobe pulse output of valve Var, Figure 4. When so triggered this valve provides a squegged pulse lasting, say, 30 60 microseconds and this is applied to the control grid of a valve V21 provided with a transformer T5 in its anode circuit. A secondary winding of such transformer provides the H. T. supply, in the form ofshort pulses, to the associated receiver l4.

Instead of employing a thermionic valve switching circuit for the cos and -sin pulse form one such mechanical relay embodiment.

In this arrangement, the amplified controlling waveform of Figure 8 (iii) is applied as an unbalanced input to the moveable tappings of the toroidal potentiometer SCP I. The quadrantal pairs of oppwed fixed tappings of the potentiometer winding are taken to opposite fixed contacts of relays RL: and RLa. The moveable contacts of such relays are connected in turn to the fixed contacts of a third relay R114 whose moveable contact is connected to the input terminal of a meter amplifier II supplying the meter 24.

In operation, the relay RL4 is arranged to be changed over from one fixed contact to the other every of the ground aerial switching cycle while the relays RL: and R10 are arranged also to be changed over every 180 of the cycle but at times which are 90 displaced with respect to relay R114. The relay operation is conveniently effected b means of a thermionic valve circuit controlled by the sine wave derived from the width-modulated back edges of the received pulses.

A modification of the arrangements described above suitable for delineating a single track is shown in Figure 12. In this arrangement two aerials Al, Bl only are used on the ground and the power from the transmitter I0 is switched to each in turn. When a distant craft is located on the perpendicular bisector b of the line joining the aerials, there is no discontinuity in the phase of the pulses received at the craft, On either side of this bisecting line, however, there is a phase discontinuity on switching whose sense; relative to a reference wave, reverses as the line is crossed. The mobile craft equipment previously described can follow such a single track without modification. The sine/cosine potentiometer should be set to 45. Two pulses of the same sign per revolution of the ground aerial switch will be selected by the valve circuits of display unit 23' and passed to the meter 25. The meter will read zero only when the pulses disappear, i. e., when the craft is located on the wanted track constituted by the bisector. line.

By this method the same equipment can be used in an aircraft for beam approach to a landing runway and for long range navigation. If Working is required only with the modified arrangement just described, the sine/cosine potentiometer can, of course, be omitted with some simplification of the mobile craft apparatus.

Figure 13 shows a further alternative ground beacon lay-out using four aerials A, B, C and D' at the corners of a square and each, supplied, as before, from a single transmitter l through a switch II and equal length feeder lines l2. The

switching sequence is A, C, B, D' and so on and the related phase changes at a remote mobile craft 1 cos 0,

/2 cos (45+o) lsin 0 and sin (454-0), where l is the diagonal length of the square. The control waveform pulses corresponding to 1 cos 0 and Z sin0 are selected as before. No modification to the mobile craft equipment'is necessary.

The arrangements described above are capable of appreciable modification in order to adapt them to difierent specific purposes. For instance the oscillator unit I! of the mobile craft equipment maybeadapted so as to be adjustable to operate at any one of a number of pulse recurrence frequencies differing by more than 2%. A number of ground beacon stations ma then operate on a common radio frequency but at different pulse recurrence frequencies. Selection of the required beacon is then effected by appropriate adjustment of the-operating frequency of the oscillator unit I]. e

I claim: 1. Radio navigation system in which the bearmitted pulses, a phase between the phase of the reference oscillation and that of the trains of transmitter pulses, and an indicator operated by the electrical outputs. 3. Radio navigation system in which the bearing of a mobile craft with respect to a fixed station is determined at said mobile craft, comprising at said station a transmitter generating trains of pulses at'constant pulse recurrence frequency, at least two spaced aerials associated with said transmitter, and means for switching the trains of pulse signals between said aerials in predetermined regular order, and at said mobile craft, means for receiving and detecting said pulse signals, a local oscillator for generating a reference oscillation, means for changing and controlling the frequency of said local oscillator to bring said reference oscillation into a predetermined phase relationship with pulse signals received from each aerial in turn, means for deriving potentials proportional to the changes in frequency of said oscillator, and an indiactor operated by said potentials.

4. Radio system for signalling between a fixed station and a mobile craft, comprising at said fixed station a transmitter generating trains of pulses at constant pulse recurrence frequency, at

' least two spaced aerials associated with said transmitter, switching means for distributing trains of pulse signals between said aerials in cyclic order, and means for modulating the charing of a mobile craft with respect to a fixed station is determined at said mobile craft, comprising at said fixed station a transmitter generatingl trains of pulses at constant pulse recurrence fre-' quency, at least two spaced aerials associated with said transmitter and means for switching the trains of pulse signals between said aerials inpredetermined regular order, and at said mobile 'craft, means for receiving said pulse signals, a phase comparing device for determining the phase relationship of the pulses received from one aerial with respect to those received from the other aerial, and an indicator operated by said phase comparing device providing a bearing indication in accordance with the particular phase relationship value.-

2. Radio navigation system in which the bearing of a mobile craft with respect to a fixed station is determined "at said mobile craft, comprising at said fixed station a transmitter generating trains of pulses at constant pulse recurrence frequency, at least two spaced aerials associated with said transmitter and means for switching the trains of pulse signals between said aerials in" predetermined regular order, and at said mobile craft, means for receiving said pulse signals, means for generating a reference oscillation at the same pulse recurrence frequency as the transpulse generator generating pulses at constant acter of the pulses in accordance with the cyclic distribution of the trains of pulses, and at said mobile craft, means for receiving and detecting said pulse signals, a local oscillator for generating a reference oscillation, means for generating a potential for changing the frequency of the local oscillator, automatic means for locking the reference oscillation to the trains of pulses received from different aerials when said reference oscillations are in phase with said trains of pulses, means responsive to the modulation carried by the pulses for selecting the different frequency changing potentials, and an indicator operated by the cyclic application-of said frequency modulated potentials.

5. Radio beacon comprising a radio transmitter generating pulses at constant pulse recurrence frequency, a plurality of spaced aerials associated with said transmitter, switching means for supplying the pulse output of said transmitter in trains of pulses to different aerials in predetermined cyclic order, and means for modulating the character of the pulses in accordance with the cyclic operation-of said switching means.

6. Radio beacon as claimed in claim 5 wherein the aerials are spaced apart in the horizontal plane and disposed in lines at right angles to one another.

7. Radio beacon comprising a transmitter generating pulses at constant pulse recurrence frequency, three aerials associated with said transmitter and arranged respectively at the corners of a right-angled triangle lying in the horizontal plane, switching means for supplying the pulse output of said transmitter in trains of pulses to said aerials in the cyclic order, the apex aerial, one base corner aerial, the apex aerial, the opposite base corner aerial and so on, and means for modulating the character of the pulses in accordance with the position of the aerial to which they are switched.

8. Radio transmitting system comprising a comparison circuit giving electrical outputs corresponding to the differences pulse recurrence frequency a plurality of spaced aerials associated with saidtransmitter, switching means for supplying the pulse output of said generator to different aerials in cyclic order, and .means for modulating the width of the pulses in accordance with the cyclic operation of said switching means.

frequency and carrying an additional modulation characteristic, a phase discriminator providingan output dependent upon the phase difference between separate trains of pulses, means controlled by the additional modulation characteristic for selecting of the output of said discriminator in predetermined cyclic order, and an indicator giving an indication dependent on the relativevalues of the output portions selected.

11. Radio receiver for a mobile craft comprising means for receiving and detecting pulse modulated radiation of constant pulse recurrence frequency, means responsive to the leading edges of the detected pulses for initiating derived pulses of predetermined time duration, a phase discriminator providing an output dependent upon the phase difference of separate trains of said derived pulses, and an indicator operated by the output of said phase comparing means.

12. Radio receiver for a mobile craft comprising -means for receiving and detecting pulse modulated radiation of constant pulse recurrence frequency, a local oscillator for generating a reference oscillation of the same pulse recurrence frequency, means for generating potentials for changing the phase of the reference oscillation into a predetermined phase relationship with difierent trains of received signals, said potentials .being proportional 'to the change of phase, and means for measuring said potentials.

13. Radio receiver for a mobile craft comprising means for receiving and detecting pulse modulated radiation of constant pulse recurrence frequency, a local variable frequency oscillator for generating a reference oscillation, means for changing the frequency of said local oscillator to bring the reference oscillation into phase with different trains of received signal pulses, and

means for comparing the different changes of frequency of the local oscillator effected.

14. Radio receiver fora mobile craft comprising means for receiving and .detecting pulse modulated radiation of constant pulse recurrence frequency, avlocal variable frequency oscillator for generating a referenceoscillation, means for changing the frequency of said local oscillator to bring the reference oscillation into phase with different trains of received signal pulses, means for locking the local oscillator in synchronism with different trains of signal, pulses in turn, and means for comparing the different changes of frequency of the "local oscillator effected.

15. Radio receiver as claimed in claim 14 including a gating circuit which suspends the operation of the apparatus except for short-periods embracing the arrival times of pulses from the ground transmitter.

16. Radio receiver as claimed in. claim 11 in-' cluding an automatic volume control circuit for regulating the gain of the signal pulse receiving means, said circuit being responsive to the amplitude of the leading edge of said pulses.

1'7. A wireless signalling system for aiding navigation of a mobile craft, comprising a transmitter for generating pulse signals, a plurality of spaced aerials, means for switching trains of pulses between said aerials in a predetermined cyclic order, and means for modulating the charactcr of the pulses in accordance with cyclic operation of said switching means, and at said mobile station, means for deriving an electrical output depending upon the difference in the transit times ofv pulses received from different aerials, indicating means, and a common signal path between said first mentioned means and said indicating means, and means responsive to the modulation of the pulses for the cyclic selection of' portions of the output of said first mentioned means through said signal path.

FREDERIC CALLAND WILLIAMS.

REFERENCES CITED file of this patent:

UNITED STATES PATENTS Date 

